1. Technical Field
The present invention relates to a gas generator for a vehicle occupant restraint, such as an air bag, and to a gas generating composition which is used in the gas generator.
2. Description of the Prior Art
A large number of gas generating compositions have been proposed for generating gas to operate vehicle occupant restraints, such as air bags and pretensioners for seat belts. For air bags, in particular, compositions which produce an inert gas, such as nitrogen, have been preferred.
Excellent results have been achieved with a solid nitrogen generating composition which comprises an alkali metal azide fuel and a metal oxidant for the fuel. Compositions comprising these materials produce, on combustion, a nitrogen gas along with other products of reaction.
The solid nitrogen generating compositions are contained within a gas generator apparatus. The size and weight of the gas generator apparatus is dependent to a large extent on the amount of gas generating material. To generate enough gas to inflate an air bag, a substantial amount of nitrogen generating composition is required. The gas generator apparatus, particularly for a driver's side air bag, must be small and light weight, so as to be adapted to fit within the vehicle steering wheel. If the amount of nitrogen generating composition can be reduced without significantly affecting the performance of the gas generator apparatus, the size and weight of the gas generator apparatus can be reduced proportionately.
U.S. Pat. No. 3,741,585 discloses a gas generating composition for inflating a vehicle crash bag. The composition comprises a metal azide and an oxidizing agent. Lithium azide (LiN.sub.3) is listed as one suitable metal azide. However, no reactions are disclosed which use this ingredient. The oxidizing agent can be a metal oxide, a metal sulfide or sulfur, a metal or organic iodide, or an organic chloride. Examples of metal oxides given in the patent are molybdenum trioxide, tungsten trioxide, lead dioxide, and vanadium pentoxide. The composition can contain 1-75 weight percent of an oxidizing agent. However, preferred compositions comprise 30 to 75 weight percent of a metal azide; e.g., 30 to 75 weight percent of sodium azide.
U.S. Pat. No. 3,895,098 discloses a gas generating composition for a vehicle occupant restraint system. The composition comprises an alkali metal azide and a metal oxide. The patent lists lithium azide as one suitable alkali metal azide. The patent calls for a slight excess of metal oxide to prevent, on combustion, the formation of alkali metal. The alkali metal, if formed, can burn spontaneously upon contact with air, and present a fire hazard. Also, an alkali metal residue can react with moisture to produce hydrogen, which is an undesirable by-product.
U.S. Pat. No. 3,931,040 also discloses the combustion of an alkali metal azide and a metal oxide to produce a nitrogen containing gas. A number of reactions involving different azides and different oxides are disclosed. One reaction equation disclosed in the patent uses lithium azide. The lithium azide is reacted with a stoichiometric amount of titanium dioxide. This reaction would be impractical for the reason advanced in the '098 patent. Specifically, without an excess of metal oxide, an alkali metal residue could form, presenting a fire hazard.
U.S. Pat. No. 4,758,287 is yet another patent that discloses the reaction of a alkali metal azide with a metal oxide. This patent is directed primarily to a method for preparing porous propellant grains. No reaction equation is disclosed in the patent involving the use of lithium azide. The patent discloses and claims the use of a stoichiometric excess of metal oxide to prevent the formation of alkali metals.